The good sealing caprocks are significant for the integrity of underground gas storage (UGS) in depleted natural gas reservoirs. The screening of parameters, weight assignment, and evaluation method are important for evaluating the sealing performance of caprocks. Many factors can affect the sealing performance of caprocks, including caprock thickness, lithology, brittleness, porosity and permeability, breakthrough pressure, etc. In this paper, the dominant factors in the sealing performance of caprocks in UGSs are systematically analyzed, and the weights of these factors are analyzed by the analytic hierarchy process (AHP). The fuzzy comprehensive evaluation method (FCEM) is applied in the sealing evaluation of caprocks in three typical underground gas reservoirs (i.e., Zhujiadun, Xu-2, and Xing-9) in China. The sandstone reservoir in the Zhujiadun gas field is only about 20 m, and the thickness of the overlying mudstone is about 600 m. The caprock of the Xu-2 gas reservoir in Zhongba gas field is well distributed and developed, and the breakthrough pressure is relatively large. The caprock of Xing-9 gas field is mudstone with a thickness of over 400 m. The results show that the breakthrough pressure and permeability are the key parameters affecting the sealing ability of caprocks, with weights of 0.4291 and 0.2157, respectively. Among these three examples of gas fields, the sealing performance of caprocks in Zhujiadun gas storage is the best. The evaluation procedure and methods proposed in this paper are valuable for the evaluation of the tightness of caprocks in depleted gas reservoirs.

Low-permeability sandstone reservoirs have been widely used as a gas storage medium worldwide. Compared with the high porosity and high permeability of sandstone, low-permeability sandstone may present different mechanical (deformation, damage or failure) and acoustic responses under cyclic loading-unloading processes caused by the high-rate injection–production of underground gas storage. In this paper, multistage triaxial loading–unloading tests with a continuously increased upper limit of stress were carried out on low-permeability sandstone under six different confining pressures. The results showed that the superposition of stress–strain curves become much denser in the process of each level of stress. Based on the variation of the elastic modulus of low-permeability sandstone under alternating loads, the mechanical behavior of low-permeability sandstone under cyclic loading is divided into three stages: cyclic hardening, stability and cyclic softening. According to the evolution of acoustic emission (AE) signal parameters, AE counts appear intensively at the initial stage of each level of stress and then gradually stabilize. The peak frequency presents the zonal distribution, which is divided into low-frequency, intermediate-frequency and high-frequency zones. Low confining pressure leads to a small b-value. The RA–AF distribution implies that the mixed tensile–shear cracks are continuously generated in low-permeability sandstone during the cyclic loading process, and the shear cracks are more obviously developed.

Depleted gas reservoirs are important natural gas storage media, thus research on the mechanical properties and damage evolution of reservoir rocks under alternating load conditions has significant practical implications for seal integrity studies. This paper conducted multi-level cyclic loading triaxial compression experiments on medium-porosity medium-permeability sandstone under different confining pressures and used acoustic emission (AE) instruments to detect the AE characteristics during the experiment, analyzing the mechanical characteristics, AE, and damage evolution characteristics. The experimental results show that after cyclic loading, the peak strength of sandstone increased by 14–17%. With the increase in the upper limit stress of cyclic loading, the elastic modulus showed a trend of first increasing and then gradually decreasing. The damage variable of rock samples rose with a rise in the upper limit stress of cyclic loading and confining pressure, and the rock damage was mostly localized at the peak stress. The AE b-value increased generally as confining pressure increased, showing that fractures occurred quicker and more unevenly at lower confining pressures. The distribution of RA-AF values shows that a sudden increase in stress causes the initiation and expansion of cracks in medium-permeability sandstone, and that tensile and shear cracks form continuously during the cyclic loading process, with shear cracks developing more pronounced. This research can provide some theoretical guidance for the long-term stable operation and pressure enhancement expansion of depleted gas reservoir storage facilities.